The present invention is related to the chemical processing arts. It finds particular application in the preparation of trans-butadiene polymers which are excellent in moldability and processability for use as additives in rubber manufacturing. It is to be appreciated that the invention is also applicable to preparation of a variety of other low molecular weight polymers.
Low molecular weight polymers are useful processing aids in rubber compounds. Polymers with unsaturation in the backbone provide superior characteristics when blended with tire elastomers since the low molecular weight polymers are able to cocure with the elastomer. The processing aid thus becomes a load bearing component of the cured matrix. Other processing aids, such as processing oils, waxes, and low molecular weight saturated polymers act as diluents and do not contribute to the strength of the cured product.
Polyisoprene and polyoctanamer (Vesetenamer(trademark)) have been used as processing aids in rubber compounds. Both, however, tend to be expensive. Polyisoprene is a viscous resin, which is difficult to handle. More recently, low molecular weight trans-polybutadiene has been found to be useful as a reactive processing aid. It has advantages in that it is less expensive to produce than other processing aids with unsaturation and it is a solid at room temperature, allowing it to be handled in pelletized form.
Butadiene polymers having a high content of trans-1,4-linkage have been produced using polymerization catalysts. Such polymers can be produced using three known techniques, as follows:
1) a preparation technique using a Ziegler catalyst containing a transition metal as the main component;
2) a preparation technique using an anion polymerization catalyst system comprising an alkaline earth metal compound as the main component; and
3) a preparation technique using a catalyst comprising a rare earth metal compound as the main component.
In the first technique, transition metals, such as nickel, cobalt, titanium, and vanadium have been found to effect a high degree of stereo-regular polymerization of a conjugated diene monomer. For example, butadiene may be polymerized using a titanium metal in the form of a tetravalent titanium metal compound, and a carrier of magnesium halide. Polymerization of isoprene using a complex catalyst of tetravalent vanadium halide and an organic aluminum has also been reported.
In the second technique, an organometal compound of a group IIA a metal is used as the polymerization catalyst. Beryllium and magnesium organometal compounds may be synthesized with relative ease. However, polymerization activity for conjugated dienes is only exhibited under special reaction conditions. In contrast, group IIA metal salts of organic acids, for example barium and strontium, when combined with other organic metal compounds are known to be effective for polymerization of conjugated diene monomers. For example, catalyst systems employing barium-di-tert-butoxide and an organomagnesium compound have been used in the polymerization of butadiene. Organic compounds of barium or strontium, in combination with an organolithium and an organic metallic compound of group IIB or IIIA metal have also been used.
In the third technique, a salt or complex of a lanthanide metal is used in combination with an organo-magnesium compound or an organotithium compound. The organomagnesium compound is a dihydrocarbyl magnesium compound, such as a dialkyl magnesium, dicycloalkylmagnesium, or diarylmagnesium compound. The rare earth element may be any of those from atomic number 57(lanthanum) to 71 (lutetium), although some are less effective than others. Organic acid salts of neodymium (e.g., Versatic acid, a synthetic acid composed of a mixture of highly branched isomers of C10 monocarboxylic acids, sold by Shell Chemicals) and an organomagnesium compound have been used to produce crystalline butadiene having high trans-linkage. Other Versatic acid salts, such as those of Di, Pr, and the like may also be used. A tri-block or radial arm block copolymer can be prepared from a styrene or styrene butadiene block and a high trans-poly-butadiene block (greater than 80% trans). The styrene or styrene/butadiene block is first prepared using a butyllithium/ dibutylmagnesium catalyst system. When the polymerization is complete, a lanthanum organic acid salt is added, followed by more butadiene. Formation of crystalline trans-butadiene polymers in a hydrocarbon solvent using a complex catalyst of organic acid salts of lanthanum or cerium and an organomagnesium compound also is known.
Low molecular weight processing aids of the types described are viscous resins at the temperatures at which they are desolventized. Because the molecular weight of the compounds formed is generally less than 100,000, and often around 40,000, desolventizing by conventional methods, such as steam desolventizing or drum drying, is not possible. Desolventizing is the removal of solvents used to facilitate the formation of the processing aids. Equipment capable of desolventizing viscous liquids is not generally found in synthetic elastomer plants, which generally handle much higher weight elastomers.
There remains a need for a new and improved method of preparing low molecular weight butadiene polymers for use as a processing aid in rubber compounds and the like, which allows the use of conventional desolventizing processes.
Briefly, the present invention provides a method for preparing a butadiene-type polymer. A butadiene monomer, optionally with another conjugated monomer, is polymerized in a suitable solvent in the presence of a catalyst to form a polymer of a first molecular weight. The polymer is reacted with a coupling agent to form a coupled polymer of a second molecular weight which is higher than the first molecular weight. The coupled polymer is treated to remove the solvent. The desolventized coupled polymer is decoupled to provide a decoupled polymer having a lower molecular weight than that of the coupled polymer.
One advantage of the present invention is that polybutadiene polymers may be formed at low molecular weight.
Another advantage of the present invention is that conventional desolventizing procedures may be used for extracting the polymer.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.